Switching amplifiers are utilized in a variety of different technologies, including motor control, switching radio frequency (RF) power amplifiers as well as audio amplifiers. The output power provided to a load is controlled by switching power switches (e.g., transistors) in the output stage of the amplifier. The output stage of the switching amplifier can be implemented as a half-bridge or a full H-bridge, such as by employing an arrangement of power transistors. The load of a switching amplifier, for example, can be a circuit containing an inductive component, such as a motor or a speaker.
Class D amplifiers employ pulse width modulation techniques to convert input signals to pulse trains, which are then used to recover input signal information from amplified pulse trains. One type modulation scheme that can be implemented by a switching amplifier is Class AD operation in which the output is modulated between two states (e.g., a high level and a low level) at a variable rate according to an input signal. Class D amplifiers can theoretically achieve a high efficiency (e.g., about 100%) implementing AD operation, assuming the use of ‘ideal’ components. A high level of output ripple voltage, however, can exist at low signal levels. For example, traditional Class D amplifiers, implementing Class AD operation, typically require large AC coupling capacitors to drive single ended loads. In addition to adding significant cost to the amplifier structure, the large capacitors can cause power limitations at upper audio frequencies. The two-state AD operation also yields lower efficiency than BD operation because of strong circulating currents that tend to occur in the case of AD operation.
Another type of switching amplifier system implements class BD operation, which was devised to overcome some of the deficiencies of Class AD operation. A Class BD amplifier generates sets of variable-width pulse trains of either positive or negative polarity at an instant signal condition. The generated signal returns to zero volts between pulses. The three-state operation is known as Class BD operation because pulses of only one polarity actually “carry” the signal at a given signal polarity, and the output is similar to the operation of a traditional Class B power amplifier.
Improved systems and methods for implementing amplifiers are desired to achieve efficient operation and low distortion at the output.